The optical recording medium (including a magneto-optical recording medium) represented by a compact disc (CD), a minidisc (MD), and a digital video disc (DVD) is widely utilized as a storage medium of music information, video information, data, programs and so on.
However, by demands for a higher sound quality, a higher picture quality, a longer operable time and a greater capacity in music information, video information, data, programs and the like, an optical recording medium (including a magneto-optical recording medium) having a still greater capacity as well as an optical recording/reproducing apparatus (including a magneto-optical recording/reproducing apparatus) for recording on and reproducing from such optical recording medium are desired.
Thus, to cope with the above-described demands, in the optical recording/reproducing apparatus (including a magneto-optical recording/reproducing apparatus), it has been attempted to reduce wavelengths of a light source, e.g. a semiconductor laser or increase a numerical aperture of a condenser lens for reducing a diameter of a light spot converged through the condenser lens.
For example, as to the semiconductor laser, a GaN semiconductor laser having oscillation wavelengths reduced from 635 nm of the conventional red color laser to 400 nm region is being put into practice, whereby the diameter of light spot is being reduced.
Moreover, as to making the wavelengths still shorter than that, for example, a far-ultraviolet solid laser UW-1010 made by Sony corporation, which continuously emits light of a single wavelength of 266 nm is sold. The diameter of light spot is thus being aimed to make still smaller. In addition thereto, the research and development of a double-wave laser of Nd:YAG laser (266 nm region), a diamond laser (235 nm region), a double-wave laser of GaN laser (202 nm region) and the like are being proceeded.
Furthermore, a near-field optical recording/reproducing system is examined, in which a condenser lens having a numerical aperture of, e.g. 1 or more is materialized by using, e.g. an optical lens with a large numerical aperture, represented by a solid immersion lens (SIL), and also an objective surface of the condenser lens is made to approach the recording medium so that a distance between them may be about a wavelength of light from a light source, for recording and reproduction.
In this near-field optical recording/reproducing system, it is important how to keep the distance between the recording medium and condenser lens in an optical contact condition.
Moreover, as a diameter of luminous flux which is emitted from a light source and incident on a condenser lens becomes small, the distance between the recording medium and condenser lens becomes extremely small, so that the shape of a condenser lens will greatly be restricted.
In this connection, a schematic structure diagram of a main part of an optical pickup having the above-described condenser lens is shown in FIG. 12.
As shown in FIG. 12, this optical pickup is provided with a condenser lens 53 composed of a first optical lens 51 in a super-hemisphere shape (a shape having an addition to a hemisphere) and a second optical lens 52 which are arranged successively from an objective side where a recording medium (an optical recording medium or a magneto-optical recording medium) 50 exists.
Both of the first optical lens 51 and the second optical lens 52 are made of glass (that of a refractive index n=2.0, or SiO2 glass of a refractive index n=1.5).
This condenser lens 53 can converge luminous flux L to irradiate the recording medium 50.
Moreover, an objective surface of the condenser lens 53, i.e. the surface of the first optical lens 51 facing the recording medium 50 is made to approach the recording medium 50, and the condenser lens 53 forms that of the above-described near-field optical recording/reproducing system.
The first optical lens 51 in the shape of a super-hemisphere has a relation t=r(1+1/n), where r is a curvature radius of the optical lens, n being a refractive index n of the optical lens, and t being a thickness of the optical lens. In this case, if the refractive index n=2.0, then t=1.5r. If the refractive index n=1.5, then t=1.667r.
Furthermore, where WD is a distance between the second optical lens 52 and the recording medium 50, which depends on a numerical aperture of the second optical lens 52, a condition t<WD must be satisfied. In other words, when the refractive index n=2.0, a condition t=r(1+1/n)=1.5r<WD must be satisfied. When the refractive index n=1.5, a condition t=r(1+1/n)=1.667r<WD must be satisfied.
Therefore, to secure a distance D between the first optical lens 51 and the second optical lens 52 appropriately and easily, it is necessary to form the curvature radius r of the first optical lens 51 as small as possible, or select its material so as to make its refractive index n as large as possible.
However, the curvature radius r of the first optical lens 51 cannot be reduced to about 1 mm or less due to a restriction on the accuracy in assembling an optical pickup.
In the near-field optical recording/reproducing system, the condenser lens 53 having a numerical aperture of 1 or more is materialized by combining two optical lenses of the first and second optical lenses 51, 52 generally arranged in turn from the objective side. However, the larger a numerical aperture becomes, the higher precision is required in assembling these first and second optical lenses 51, 52 and also it is required to keep this high precision against a change of surroundings.
Moreover, if a curvature radius of the optical lens is too small, it becomes impossible to heighten the precision in assembling the condenser lens 53 composed of the two optical lenses 51, 52. Accordingly, it is impossible to make the curvature radius r of the first optical lens 51 smaller than about 1 mm.
Furthermore, because glass has been used as material of the optical lens in the past, the refractive index n of optical lens could not exceed about the aforesaid 2.0.
Therefore, the lower limit of thickness t of the first optical lens 51 was about 1.5 mm and making it smaller than that was impossible.
When SiO2 glass is used, the limit of refractive index n of optical lens is the aforesaid 1.5 or so, the limit of thickness t of the first optical lens 51 being 1.667 mm or so, and making it smaller than that being impossible.
On the other hand, to realize a high-density recording in the near-field optical recording/reproducing system, the same as the conventional optical recording/reproducing system, it is necessary to reduce a size and area of a condensed light spot irradiating a recording medium by shortening a wavelengths of light emitted from a light source and increasing a numerical aperture of a condenser lens. In this connection, because an area of the condensed light spot is inversely proportional to the square of a numerical aperture of condenser lens, to realize a high-density recording in the near-field recording/reproducing system, it is effective to increase the numerical aperture of condenser lens.
In the structure where the first optical lens 51 is a super-hemispherical optical lens shown in FIG. 12, a numerical aperture NA of the near-field condenser lens 53 can be expressed by NA=(a numerical aperture of the second optical lens 52)×(a refractive index n of the first optical lens 51)×(a refractive index n of the first optical lens 51).
As described above, because glass has been used as materials of the first and second optical lenses 51, 52 until now, a refractive index n of the first optical lens 51 could not exceed about 2.0. Thus, where a numerical aperture of the second optical lens 52 is,. e.g. 0.45, the numerical aperture NA of near-field system condenser lens 53 becomes NA=0.45×2.0×2.0=1.8 and it was impossible to increase the numerical aperture NA more than that value.
Also, when SiO2 glass is used as materials of the first and second optical lenses 51, 52 because the limits of refractive index n of the first optical lens 51 is about 1.5, where a numerical aperture of the second optical lens is, e.g. 0.45 as well, the numerical aperture NA of near-field condenser lens 53 becomes NA=0.45×1.5×1.5=1.0 and it will be impossible to increase the numerical aperture NA over that value.
Accordingly, with the conventional near-field condenser lens 53 made of glass materials, there is the limits to realize a high-density recording.
Furthermore, up to now, optical lens materials for 420 nm wavelengths or less, having a low light absorption as well as a cubic crystal characteristic fit for the aforesaid solid immersion lens (SIL) has been unclear.
In order to solve the above-described problems, the present invention provides: an optical lens having a high refractive index and low light absorption characteristic in a ultraviolet-wavelengths region; a condenser lens composed of the optical lens, fit for the near-field optical recording/reproducing system; an optical pickup that includes the condenser lens and can reduce the condensed light spot irradiating a recording medium and also manage to make the recording medium higher in recording density and greater in capacity; and an optical recording/reproducing apparatus that comprises such optical pickup and can perform a high-density optical recording and reproduction.